Japanese Patent Application Publication No. 2016-446717 discloses a switching circuit that switches two IGBTs (abbreviation of Insulated Gate Bipolar Transistor) connected in parallel. This switching circuit switches the two IGBTs at same time when current flowing in a parallel circuit thereof is high. In this case, the current flows by being distributed to the two IGBTs, and load applied to each IGBT can be reduced. Further, in this switching circuit, one of the IGBTs is turned off at an off-timing when the current flowing in the parallel circuit is small. The other of the IGBTs is maintained in a normally off state, or is turned off before the off-timing. In this case, a turn-off loss is suppressed by turning off the one of the IGBTs at the off-timing.
In the technique of Japanese Patent Application Publication No. 2016-146717, a control method is switched according to the current at a time when the parallel circuit was previously in an on-state, or a prediction value calculated based on such current. However, in this method, the current that would flow the next time the parallel circuit is turned on cannot be accurately be predicted, and it was difficult to suitably switch the control method according to the current. Due to this, there had been a case where high current flows in the parallel circuit when one of the IGBTs is in the on-state, and excessively large load was applied to the IGBT that is in the on-state. In view of the above, the description herein provides a technique capable of accurately switching a control method according to current flowing in a parallel circuit.
A switching circuit disclosed herein may comprise a wiring; a parallel circuit arranged on the wiring, and including a first IGBT and a second IGBT connected in parallel; a controller configured to receive a signal indicating a turn-on timing and a turn-off timing and switch one or both of the first and second IGBTs according to the signal. The controller is configured to turn on both of the first and second IGBTs at the turn-on timing. The controller is configured to execute a first control in which one of the first and second IGBTs is turned off before the turn-off timing and the other of the first and second IGBTs is turned off at the turn-off timing in a case where current flowing through the parallel circuit when both of the first and second IGBTs are in an on-state is equal to or lower than a threshold value. The controller is configured to execute a second control in which both of the first and second IGBTs are turned off at the turn-off timing in a case where the current flowing through the parallel circuit when both of the first and second IGBTs are in the on-state is higher than the threshold value.
In this switching circuit, the controller turns on both the first and second IGBTs at the turn-on timing. At the turn-on timing, it is difficult to predict the current that would flow in the parallel circuit. However, since both the first and second IGBTs turn on at this stage, the current flows by being distributed to the first and second IGBTs respectively. Accordingly, even if the current flowing in the parallel circuit is high, excessively large current is suppressed from flowing in each of the IGBTs. With the current starting to flow in the parallel circuit at the turn-on timing, control according to the current flowing in the parallel circuit becomes enabled.
In the case where the current flowing in the parallel circuit is equal to or lower than the threshold when both the first and second IGBTs are in the on-state, the controller thereafter performs the first control. In the first control, the one of the first and second IGBTs (which may hereinbelow be referred to as a preceding IGBT) is turned off before the turn-off timing, and the other of the first and second IGBTs (which may hereinbelow be referred to as a subsequent IGBT) is turned off at the turn-off timing. Since the preceding IGBT is turned off first, the current flows thereafter by being accumulated in the subsequent IGBT. However, since the current flowing in the parallel circuit is small, excessively large current can be suppressed from flowing in the subsequent IGBT. Further, since the subsequent IGBT turns off at the turn-off timing, which is after the preceding IGBT had turned off, a turn-off loss is thereby suppressed.
When the current flowing in the parallel circuit when both the first and second IGBTs are in the on-state is higher than the threshold value, the controller performs the second control thereafter. In the second control, both the first and second IGBTs are maintained in the on-state until the turn-off timing, and both the first and second IGBTs are turned off at the turn-off timing. Both the first and second IGBTs are maintained in the on-state over an entirety of a period from the turn-on timing until the turn-off timing when the current flowing in the parallel circuit is high. Due to this, the high current flows by being distributed to both the first and second IGBTs, and load on each IGBT can be reduced.
As above, in this switching circuit, by turning on both the first and second IGBTs at the turn-on timing, the high load is prevented from being applied to each IGBT even in the case where high current flows in the parallel circuit at the turn-on timing. Further, since detection of the current flowing in the parallel circuit is enabled during when both the first and second IGBTs are in the on-state, one of the first and second controls can appropriately be performed according to the current flowing in the parallel circuit. Thus, the high load can be prevented from being applied to each IGBT when the current flowing in the parallel circuit is high, and the turn-off loss can be suppressed when the current flowing in the parallel circuit is low.